012-07124B
9/99
$1.00
Instruction Sheet
for the PASCO
Model ES-9078
BASIC ELECTROMETER
Introduction
The Model ES-9078 is a voltmeter used for direct
measurements of voltage and indirect measurements
of current and charge. Because of its high (“infinite”)
impedance of 1014 ohms, it is especially suited for
measuring charge in electrostatic experiments. It has a
sensitivity nearly 1000 times that of a standard goldleaf electroscope, a center-zero meter that directly
indicates charge polarity, and measures charges as
low as 10-11 coulombs.
With these features, you’ll find that your electrostatics
demonstrations and labs are easier to perform and,
with quantitative data, are more informative.
The electrometer is powered by four AA-alkaline
batteries, easily replaced by opening the back casing
of the electrometer.
When replacing batteries, do not touch any
of the components or wires in the integrated
circuit panel; the components and
wires are all static sensitive.
EQUIPMENT
Parts included:
• Basic Electrometer (ES-9078)
• Associated Cables, four (4)
Additional parts required:
• Faraday Ice Pail (ES-9042A)
• Proof Plane
• Basic Variable Capacitor (ES-9079)
• Pasco Computer Interface (Model 750, 500 or
300)
© 1999 PASCO scientific
Basic Electrometer
012-07124B
Operation
2. Connect the test lead to the input connector of
the electrometer.
The controls on the front panel of the
electrometer are explained in Figure 1.
Whether you’re using the electrometer to
measure voltage, current, or charge, the
following procedure should be followed each
time you turn on the electrometer.
3. Connect the ground post of the electrometer to
an earth ground.
4. Push the power button ON. One of the range
switch LEDs will blink twice in quick
succession.
More information on making accurate
measurements is provided in the sections that
follow.
5. To zero the meter, press the ZERO button.
You’re now ready to use the electrometer to
measure charge, current or voltage.
6. Set the range switch to the desired voltage range.
The range setting refers to the voltage input
required to produce a full-scale meter deflection
(e.g., a setting of 30 means that a full scale meter
deflection indicates a voltage of 30 volts).
Warning:
1. Never use the electrometer for
measuring potentials more than 100 volts.
2. Never connect the electrometer to an
electrostatics generator, such as a Van de
Graff generator or a Wimhurst machine.
Important Points for General
Operations:
3. Avoid touching the input leads until you
have grounded yourself to an earth ground.
A person walking across a rug on a cool
dry day can easily acquire a potential of
several thousand volts.
1. Between measurements, always press the
Zero button to discharge all excess charge from
the electrometer. Shorting the test leads together
is insufficient. There may still be stray charges
within the electrometer circuitry. Note that on
the most sensitive scale (3 volts), the meter may
not return to exactly zero when the Zero button
is pressed. This is normal and will not affect the
accuracy of the reading.
meter
display
zero button,
ground and
remove excess
charges
select voltage
range
(3,10,30,100V)
output signal
to interface
100 V
mechanical zero
adjust screw
(adjust with
power off)
2. DO NOT use the mechanical zero adjust screw
to set the meter to zero, unless power is off.
connect test
lead here
3. For good results, it is essential that the
electrometer be connected to an earth ground (a
water pipe or the ground wire from a 120 VAC
socket). Only an earth ground provides a
sufficient drain for excess charges that may
build up during an experiment. It is also helpful
if the experimenter is grounded. Just touch one
hand to a good earth ground just before, or
during measurements.
connect to
earth ground
push to turn ON/OFF
Figure 1. Electrometer panel controls
Setup Procedure
1. Before turning on the electrometer, check that
the meter reads zero. If not, turn the Mechanical
Zero Adjust screw, located just below the meter
face, until it does.
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012-07124B
Basic Electrometer
Measuring Voltage
the electrometer reading will generally remain
relatively unchanged. This is because the total
capacitance is only negligibly affected by the proof
plane. This may not always be the case, however.
The electrometer can be thought of as an
infinite impedance voltmeter. To show that the
electrometer is like a voltmeter, connect the
leads to the battery, set the range (3, 10, 20, or
100 volts full scale), and read the voltage on the
meter.
Any charge measurement with the electrometer is
indirect, based on the knowledge that the amount
of charge on an object is proportional to the
potential difference. The readings of the
electrometer will always be of volts, not of
coulombs. The polarity of the voltage, however,
directly shows the type of charge being sampled. If
you need quantitative rather than qualitative charge
measurement, values for the charge can be
computed according to the relationship Q = CV,
where V is the voltage across a known capacitor of
capacitance C .
Measuring Charge
Induction Charges
Under most conditions, the best way to measure
charge is by induction. Use a proof plane and a
Faraday ice pail, such as those included with
PASCO’s Demonstration Electrostatics System, and
shown in Figure 2. The proof plane is simply a
small conductive disk mounted on an insulating
handle. To sample the charge distribution on a
charged object, simply touch it with the proof
plane, then place the proof plane inside the inner
cylinder of the ice pail, without touching the
cylinder. A charge of equal magnitude and sign is
induced on the surface of the ice pail and can be
read by the electrometer. By always using the
proof plane and the ice pail, the capacitance will be
the same for all your measurements, and the charge
on the proof plane will always be proportional to the
voltage reading of the electrometer.
shield
The electrometer can be thought of as an infinite
impedance voltmeter in parallel with a capacitor, CE ,
as shown in Figure 3. CE represents the internal
capacitance of the electrometer, plus the capacitance
of the leads and the capacitance of the Ice Pail, if
being used.
CE
Electrometer
Voltmeter
ice pail
black
clip
proof
plane
Figure 3. Ideal Schematic of the Electrometer
electrometer
red
clip
When a charged object is placed across the
electrometer leads (or in the ice pail), a voltage V
displays on the meter. If the value CE is known, the
value of the charge can be calculated as Q = C EV.
The capacitance of the electrometer alone is around
27 pF. However, if the sampled object adds
significant capacitance, the situation becomes as
shown in Figure 4.
insulator
Figure 2. Using a Faraday Ice Pail
Contact Charges
Charges can also be measured by contact. If you
touch the inner cylinder of the ice pail with a
charged object, for example a charged proof plane,
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Basic Electrometer
012-07124B
CE
Voltmeter
connected across the leads of the electrometer, it is
connected in parallel with the internal capacitance
of the electrometer, CE. The total capacitance
becomes C + CE. The known capacitor discharges
across the electrometer and a voltage, VE, is read.
Since the total charge in the system is still just the
charge of the known capacitor, we know that
Cext
Capacitance of
object connected to the
Electrometer
Electrometer
Figure 4. Change in capacitance due to a charged object
CV = C + CE VE .
To accurately calculate the amount of charge, total
capacitance CE + Cext must be determined. In any
case, for quantitative measurements of charge, you
need to have an accurate value for CE (Cext >>CE) ,
unless you are certain that the object you are
measuring has a high enough capacitance to
disregard CE. The capacitors in the PASCO RC
Network (ES-9053), for example, are high enough
that CE need not be considered. This is not true,
however, when using the parallel plate variable
capacitor (ES-9079).
1. Obtain a low leakage capacitor (polypropylene,
or air dielectric) of known value C, around 30
pF.
2. Charge the capacitor with a known voltage V,
CE =
C V – VE
VE
not higher than 100V (the limit of the
electrometer).
3. Remove the charged capacitor from the power
supply used to charge it, being careful not to
ground it in any way, so as not to remove the
charge.
The following process shows you how to measure an
experimental value of the capacitance of the
electrometer, any cables connected to it, and the ice
pail.
4. Connect the charged capacitor across the
electrometer input leads. (Or across the pail and
shield of the ice pail, if you want to include its
capacitance value). Note the voltage VE
indicated by the electrometer.
Measuring the Capacitance of the Electrometer
and Connectors
For all experiments, the electrometer can be thought of
as a voltmeter connected in parallel with a
capacitor, as shown in Figure 2. The capacitor CE
represents the internal capacitance of the
electrometer, plus the capacitance of the leads and
the ice pail.
5. Calculate the total capacitance:
Measuring Current
The electrometer can be used for indirect current
measurements.
Use this procedure to measure a precise value of
the capacitance provided by the electrometer and
all instruments connected to it. If you are interested
in qualitative, rather than quantitative experiments,
this procedure is not necessary.
Connect the electrometer leads across a known
resistance in the circuit and measure the voltage.
Use Ohm’s Law V = IR to determine the current.
Due to the electrometer’s exceedingly high input
impedance, the effect of the electrometer in the
circuit will be negligible in most circuits. However,
the voltage drop across the resistor must be within
the voltage range of the electrometer (100 V
maximum).
When a known capacitor C, is charged by a
known voltage V, the charge in it is given by
Q = CV. If the known charged capacitor is
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012-07124B
Basic Electrometer
Specifications
If it is not convenient to hook the electrometer
across a known resistance in the circuit, a precision
resistor can be connected between the input leads
of the electrometer. The circuit can be broken, as
for connection to a standard ammeter, and
connected in series with the precision resistor.
Again, measure the voltage across this resistor and
calculate the current. (The problem with this
technique is that the resistance must be high
enough to measure a voltage drop, but low enough
so it doesn’t significantly affect the current through
the circuit).
ES-9078 Basic Electrometer:
Voltage input ranges:
3, 10, 30, 100 volts full scale
Input resistance:
approximately 10^14 ohms
Input capacitance:
approximately 27 pF
Accuracy:
-
Analog Meter: +/ 3%
ScienceWorkshop output: +/
-
1%
Signal output: Fixed at -100 to + 100 volt range
(Automatically scaled in DataStudio
and ScienceWorkshop)
Using a Computer with Science Workshop
Interface
Batteries:
4 - “AA” (Alkaline recommended)
Battery life:
approximately 75 hours
Note: This instruction sheet assumes the
user is familiar with ScienceWorkshop® or
DataStudio™. Users can gain familiarity by
working through the tutorials provided in
ScienceWorkshop or DataStudio’s Online
Help.
The signal output connector on the ES-9078
Electrometer provides an output signal for the Science
Workshop interface. This provides an output
signal for displaying data in analog meters, tables,
graphs or oscilloscopes, making data analysis easy.
It is important to remember that the signal output on
the ES-9078 has a fixed scale of ±100 volts. Changing
the range switch on the electrometer has no effect on
this scaling. By choosing the Electrometer icon in
DataStudio™ or ScienceWorkshop®, displays are
shown correctly scaled.
The exclamation point within an equilateral
triangle is intended to alert the user of
important operating and safety instructions
that will help prevent damage to the
equipment or injury to the user.
Because of limited resolution on this output, it may be
useful to apply gain within the ScienceWorkshop
interface to measure voltages of 10 volts or less.
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Basic Electrometer
012-07124B
Technical Support
Feedback
Contacting Technical Support
If you have any comments about the product or
manual, please let us know. If you have any
suggestions on alternate experiments or find a
problem in the manual, please tell us. PASCO
appreciates any customer feedback. Your input helps
us evaluate and improve our product.
Before you call the PASCO Technical Support
staff, it would be helpful to prepare the following
information:
➤ If your problem is with the PASCO apparatus,
note:
­ Title and model number (usually listed on the
label);
To Reach PASCO
­ Approximate age of apparatus;
For technical support, call us at 1-800-772-8700
(toll-free within the U.S.) or (916) 786-3800.
fax:
(916) 786-3292
e-mail:
techsupp@pasco.com
web:
www.pasco.com
­ A detailed description of the problem/sequence
of events (in case you can’t call PASCO right
away, you won’t lose valuable data);
­ If possible, have the apparatus within reach
when calling to facilitate description of
individual parts.
➤ If your problem relates to the instruction
manual, note:
­ Part number and revision (listed by month and
year on the front cover);
­ Have the manual at hand to discuss your
questions.
Warranty
Limited Warranty
postage or freight prepaid. (Damage caused by
improper packing of the equipment for return shipment
will not be covered by the warranty.) Shipping costs for
returning the equipment after repair will be paid by
PASCO scientific.
PASCO scientific warrants the product to be free from
defects in materials and workmanship for a period of
one year from the date of shipment to the customer.
PASCO will repair or replace, at its option, any part of
the product which is deemed to be defective in material
or workmanship. The warranty does not cover damage
to the product caused by abuse or improper use.
Determination of whether a product failure is the result
of a manufacturing defect or improper use by the
customer shall be made solely by PASCO scientific.
Responsibility for the return of equipment for warranty
repair belongs to the customer. Equipment must be
properly packed to prevent damage and shipped
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Address:
PASCO scientific
10101 Foothills Blvd.
Roseville, CA 95747-7100
Phone:
FAX:
e-mail:
Web site:
(916) 786-3800
(916) 786-8905
techsupp@pasco.com
www.pasco.com